The most common method of satellite communication is by the use of so called bent-pipe repeater system. A bent pipe repeater system uses microwave frequency converters, or receivers, for direct conversion from the uplink frequencies to the downlink frequencies. These frequency converters are usually single conversion equipment, which utilize an external or internal fixed local oscillator (LO) frequency. Usually the frequency conversion is either done for a single channel application, or carrier, or more commonly for a block of channels, or multi-carriers. Of course, a channel may consist of one or more sub-channels. Consequently, a carrier may consist of one or more sub-carriers. A carrier is either modulated or un-modulated.
Such repeater systems are limited in flexibility, and do not allow the user/operator to change the frequency of the up- and downlinks for adaptation to new traffic and/or changes in traffic. This limitation can be overcome by using a large number of frequency converters, each having a unique local oscillator frequency, in which case each of these can be addressed by using a switching arrangement. This arrangement is used in most system today. However, such an arrangement directly leads to a couple of drawbacks: the number of frequency converters increases, the switching network employed introduces great signal losses which need to be compensated for by having higher output power capability of said frequency converters and also by having lower noise contribution from channel amplifiers and frequency converters in the satellite payload.
It is possible to replace the fixed local oscillator by a synthesized one, and thus making it possible to address any wanted LO frequency of interest. However, it is not possible to change the local oscillator frequency in an arbitrary fashion due to the generation of unwanted spurious output signals. Harmonics of the LO frequency that will fall within the operating channel bandwidths when employing a single frequency scheme are particularly detrimental to system performance.
These drawbacks have made it technically and economically very difficult to increase the flexibility of a bent-pipe satellite communication repeater system.
EP patent application no. 0 712 104 relates to a multi-channel radio frequency signal transponder system used in communication satellites. The system includes a frequency channel-converter unit for implementing dual frequency conversions. The system down-converts the received RF signals to intermediate frequency signals which are later up-converted to the required transmission frequency signals. The frequencies can be varied using a low phase-noise phase lock loop (PLL).
U.S. Pat. No. 6,973,121 provides a method and system for receiving and transmitting modulated signals using a repeater. Further, the patent discloses how to control the repeater output frequency using modulator and demodulator circuits and a repeater system-based clock. Additionally, a tuner is provided between the repeater system input and demodulator for down-converting the received frequency to the intermediate frequency. Similarly, a tuner is provided between the modulator output and system output for up-converting the frequency to the transmission frequency. The modulator and demodulator circuits comprise numerically-controlled oscillators/PLLs, frequency mixers, and feedback circuits.
However, there is still a need for increasing flexibility with respect to frequency for satellite-based repeater systems without introducing signal degradation that limits the use of the repeater system.